Electrostatic headphone

ABSTRACT

An electrostatic headphone speaker assembly includes a push-pull electrostatic driver operated off a coupling transformer, with the transformer secondary also supplying a polarizing circuit for the driver diaphragm. For enhanced operation there is a large resistance in the polarizing circuit to provide a long time constant, and the transformer leakage inductance is resonated with the driver input capacitance at 13 kHz.; and for a stereo headphone set with two transformers provided with ground jumpers, an isolation network is provided to decouple the transformer secondaries.

Unite States Patent [72] Inventor Martin Lange, Jr.

River Hillls, Wis. [21] Appl. No. 84,919 [22] Filed Oct. 28, 19711 [45]Patented Jan. 41, 1972 [73] Assignee Koss Electronics, Inc.

Milwaukee, Wis. Continuation 01 application Ser. No. 785,166, Dec. 19,1968, now abandoned. This application Oct. 28, 19711, Ser. No. 84,919

[54] ELECTROSTATIC HEADPHONE 11 Claims, 2 Drawing Figs.

[52] 11.8. C1 179/111 [51] int. Cl H04! 19/00 [50] Field ofSearch..179/1.3, 1.3 PS, 111, 111 E; 325/492,494

[56] References Cited UNITED STATES PATENTS 1,622,039 3/1927 Lee 179/1111,928,503 9/1933 Reisz 179/111 3,084,229 4/1963 Selsted 179/1 1 13,417,334 12/1968 Banovic 325/492 FOREIGN PATENTS 1,147,423 5/1966 GreatBritain 179/1 1 1 OTHER REFERENCES Lucltett- New Radio Steals Its PowerFrom The Air- Popular Science- Apr. 1958, pp. 108 & 109.

Selsted- The Electrostatic Earphone Journal of the Audio EngineeringSociety- Apr. 1961, Vol. 9, No. 2, pp. 145- 147 Primary ExaminerRalph D.Blakeslee Anorneys-Allan W. Leiser and Arthur H. Siedel ABSTRACT: Anelectrostatic headphone speaker assembly includes a push-pullelectrostatic driver operated off a coupling transformer, with thetransformer secondary also supplying a polarizing circuit for the driverdiaphragm. For enhanced operation there is a large resistance in thepolarizing circuit to provide a long time constant, and the transformerleakage inductance is resonated with the driver input capacitance at 13kHz; and for a stereo headphone set with two transformers provided withground jumpers, an isolation network is provided to decouple thetransformer secondaries.

PATENTEB JAN 4 I972 INVENTOR TIN LAN6E,JR.

RIGHT /7 v j? K4 3L MAR ATTORNEY ELECTROSTATIC HEADPHONE BACKGROUND OFTHE INVENTION This invention relates to a high fidelity speaker assemblythat includes an electrostatic driver as opposed to an usual dynamicspeaker, and that is particularly satisfactory for headphones. Headphoneunits have been widely used in high fidelity equipment, and offer theimportant advantages, for example, of a controlled listeningenvironment, a high degree of isolation against extraneous noise, andlow power requirements which result in an extremely high efficiency andlack of distortion. Similarly, electrostatic drivers have been used inspeaker systems, and offer the known advantages among others ofextremely smooth response and a very high degree of efficiency.

Heretofore, however, there have been practical difficulties in usingelectrostatic drivers, particularly in a headphone unit. A prime problemin this regard is providing the necessary high polarizing voltage forthe movable diaphragm of the driver. Using commonly accepted techniques,this would require a separate power source which would be highlyundesirable for a headphone unit and is also unsatisfactory withstandard speakers enclosures.

SUMMARY OF THE INVENTION This invention contemplates a speaker unitincorporating an electrostatic driver in which a polarizing voltage issupplied by a coupling transformer secondary thus eliminating thenecessity for a separate power source. It is the general object of theinvention to provide an assembly of this type which is extremelyefficient and offers pure response while being of minimum size andcomplexity. Another object is to provide a stereo assembly incorporatingtwo units which provide the foregoing advantages and in which there isspecial provision to insure stereo separation. The invention isparticularly suited for headphones, but is also desirable for standardenclosures.

Other objects and advantages will appear from the description of apreferred embodiment which follows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary, somewhatschematic view in cross section of a single headphone formed accordingto the invention, and

FIG. 2 is a schematic electrical circuit diagram of a stereo headphoneset incorporating the unit of FIG:- ll.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. I, theheadphone unit shown includes a cup designated generally by thereference numeral 1 which is made of any suitably rigid plasticmaterials. The cup 1 includes a rear housing 2, a spacer ring 3, and afront plate assembly i which faces the users ear and mountselectrostatic driver elements as will be described. Assembly screws 5extend rearwardly through the plate 4 and spacer 3 and are received inthe housing 2 to connect the three cup elements together. The forwardedge of the ring 3 is cut away to define, with the plate 4, a groove 6which receives the rear or connecting edge of a typical liquid filledring or cushion 7. The cushion 7 is of course intended to rest againstthe users head around his ear to form a substantially complete sealagainst extraneous noise and to cushion vibrations.

The ring 3 is provided with a radially inwardly extending flange 8 towhich a rigid baffle 9 formed of any suitable material is attached bymeans of assembly screws 10 which extend through the plate 4 and throughtubular spacers 111. A coupling transformer 12 extends through anopening in the baffle 9 and is connected thereto by means of bolt-spacerassemblies 13. The various other circuit components which will bediscussed below are also mounted on the rear of the baffle 9 but havenot been shown in FIG. 1 for the sake of simplicity and clarity.

The front plate 4 is split to define a rear portion 14 and a frontportion lib, both of which are preferably provided with cross braces tohave a generally eight-shaped configuration, as viewed from the front,providing matching upper and lower openings therethrough. Between theportions 114 and 15 are oppositely spaced, rigid front and rear driverplates 16 and a flexible, intermediate diaphragm M. The plates 16 anddiaphragm are suitably glued in place and held in spaced relationship bygaskets l8, and together constitute a push-pull electrostatic driveradapted to face the ear of a user. The plates M are formed of a suitablyrigid material which is electrically conductive but acousticallytransparent-preferably perforated brass. The diaphragm I7 is formed of asuitable polyurethane film which is coated with an electricallyconductive material and secured in place under suitable tension.Suitable leads (not shown) lead from the plates 16 and diaphragm 17 sothat they can be connected into the circuits that will be described.

A perforated rigid grille 19 is received within the front plate portionopenings in front of the front plate 16, and felt layer 20 is in theopenings of the rear plate portion 14 behind the rear plate 16. Betweenthe baffle 9 and plate 4 is a layer 21 of a suitable foam material, anda second mass of foam 22 is disposed in the rear of the housing 2, theseelements serving to damp and control response in known fashion.

Although relatively unimportant and optional structural details have notbeen shown or referred to, the foregoing description will enable thoseskilled in the art to understand one manner in which an electrostaticheadphone of the type contemplated herein can be constructed. Variousmodifications known to the art are possible, however, it being necessaryfor the purposes of the invention only that there by an electrostaticdriver and a coupling transformer within a cup or other enclosure withthe driver being adapted to face out of the enclosure.

FIG. 2 is a circuit diagram for a stereophonic headphone or other unitincorporating two units like that shown in FIG. 1, in which the twounits would be mounted in usual manner on a suitable headband. With theexception of certain electrical components which will be apparent, thetwo units are identical, and like reference numerals have been used toidentify corresponding elements. There are electrical connectionsbetween the two units, and conductors to effect these are of coursepassed through the headband or simply extend between the units if otherenclosures are used.

The primaries of the two coupling transformers 12 are connected tosignal sources 23, 23, which are intended in the preferred embodiment tobe the two output channels of a stereophonic amplifier. The secondariesof the two transfor mers 12 are connected to respective sets of driverplates 16 to develop charges thereon which will cause movement of thediaphragm 17 to produce sound, all as is well known in the art. Thereare ground jumpers 24 across the transformers 12, and the twotransformers have a common ground as indicated at 25.

The left-hand unit as seen in FIG. 2 includes a polarizing circuit whichsupplies both units; and, again, the components and connections formingthis circuit will be mounted on the baffle 9 of this unit. The circuitincludes voltage tripler means designated generally by the referencenumeral 26 which comprises a network made up of three capacitors 27 andthree diodes 28, the operation of which is known to those skilled in theart. The tripler network 26 feeds a storage condenser 29, the charge onwhich is limited by a set of stacked zener diodes 30, a resistor 31 ofrelatively low resistance value being used as a filter and currentlimiter for the zener diodes 30. The output of the storage condenser 29is fed through lines 32, 32 to the two diaphragms 17 will resistors 33,33 of relatively high resistance value being interposed in respectivelines.

An indicator line 34 leads from the tripler network 26 through a voltagereducing resistor 35 to indicator lamps 36 which serve to indicatewhether the unit is functioning. No indicator lamp 36 is shown in FIG.1, but it will be appreciated that the lamp will be mounted on andvisible through a suitable opening in the cup 1 in known fashion.

An isolation network including a resistor 37 of relatively highresistance value and capacitor 38 are interposed between the rightpolarizing line 32' and the secondary of the righthand transformer 12.Because of the ground jumpers 24 the two transformers are effectivelycoupled, and the isolation network serves to isolate and decouple thetwo transformer secondaries to prevent loss of stereo effect.

It will be appreciated that the voltage tripler network 26 functions tocharge the storage condenser 29 which in turn provides a polarizingvoltage to the diaphragm 17. With properly designed driver units, thereis very little charge leakage, and thus the polarizing circuitry has nosignificant effect on overall output. Under normal conditions it can besufficiently supplied by transient peak signals which would notordinarily appear as output in any event.

Although the values of the several circuit components are variable tosome extent, certain parameters have been found to be desirable. Withusual headphone elements, a charge of approximately 500 volts on thestorage condenser 29 has been found to provide sufficient polarizingvoltage. To supply this, it has been found most satisfactory to use a1:60 transformer which, in conjunction with the tripler network 26provides a 1:180 step-up ratio. The tripler network 26 is nottheoretically necessary and sufficient step-up could be provided by thetransformer alone; but this is not feasible as a practical matterbecause of size and other limitations and the tripler network has beenfound highly desirable for operation within practical limits.

The value of the polarizing resistances 33, 33 should be sufficient toprovide a long time constant which provides greater linearity anddynamic stability for the diaphragm. In the preferred embodiment, thevalues of the resistors 33, 33' are approximately 32 megohms; but in anyevent it is desirable to have sufficient resistance value to provide anRC time constant which is at least four times greater than the timeinterval of one-half cycle of the lowest frequency to be radiated.Assuming a lowest frequency of about 20 Hz., therefore, the timeconstant should be at least approximately 0.1 sec.

In a headphone unit of the type shown, the acoustic shunt capacitance ofthe volume of air enclosed between the driver diaphragm l7 and the usersear presents a problem, and this space must be reduced to an absoluteminimum. Even so, however, there is a tendency to have a cutoff inresponse at approximately 13 kHz. with a headphone of; the general typeshown. In the preferred embodiment, any loss in response is circumventedby selecting transformer and driver elements so that the transformerleakage inductive reactance resonates with the driver input capacitanceat about 13 kHz. to restore proper level. The actual selection ofcomponent characteristics will include many variables, but these and thenet effect desired will be understandable and readily attainable bythose skilled in the art.

The preferred embodiment shown and described provides a headphone unitand stereo assembly with an extremely high degree of efficiency anduniformity of response. A prime feature in providing a desirable unit isthe use of the polarizing circuit which is supplied by the couplingtransformer secondary, and additional particularly important featuresinclude the provision of the polarizing resistances 33, 33 to provide adesirable time constant, the isolation network to decouple thetransformer secondaries and prevent the loss of stereo effect, and theresonance of the leakage inductance and input capacitance atapproximately 13 kHz. Although a preferred embodiment of the inventionhas been shown and described, however, it will be apparent thatmodifications in construction and use might be made without departurefrom the actual spirit of the invention. The invention is not,therefore, intended to be limited by the showing herein or in any othermanner except as many specifically be required.

I claim:

l. A headphone speaker unit comprising: an enclosure with a speakeropening; an electrostatic driver having opposite plates and a central,movable diaphragm, said driver being mounted in the enclosure to beadapted to face outwardly through the speaker openings; a couplingtransformer mounted in the enclosure that has a primary adapted to beconnected to an audio signal source and a secondary connected to thedriver plates; and a polarizing circuit leading from the secondary toprovide a polarizing voltage for the diaphragm, wherein said polarizingcircuit includes a voltage multiplier network having an input connectedto said coupling transformer secondary and an output connected to saiddriver diaphragm.

2. A unit according to claim 1 wherein the polarizing circuit includes arelatively high value resistance element connected between the voltagemultiplier output and driver diaphragm to provide a time constant ofapproximately at least 0. 1 sec.

3. A unit according to claim 1 wherein the voltage multiplier networkfeeds a storage capacitor which is provided with voltage limiting meansdesigned to limit the charge thereon to a predetermined maximum voltage.

4. A speaker unit as recited in claim 1 wherein the leakage inductivereactance of the coupling transformer secondary and the inputcapacitance of the driver are turned to resonate at a frequency aboveapproximately 13 kHz. to enhance the high-frequency response of thespeaker unit.

5. A stereophonic headphone assembly for use with a stereophonic signalsource, said assembly comprising: a pair of headphone units each ofwhich includes a cup, an electrostatic driver that includes oppositeplates and a central diaphragm and that is mounted in the cup enclosureto be adapted to face a users ear, and a coupling transformer that has aprimary adapted to be connected to an audio signal source and asecondary connected to the driver plates, the primaries of the twotransformers being adapted to be connected to respective channels of thestereophonic source, the two transformers being provided with groundjumpers leading to a common ground connection; a polarizing circuit inthe cup of one unit that is connected to the transformer secondarytherein to generate a polarizing voltage and has output lines leading tothe diaphragms of both drivers; and an isolation network that includes aresistance and a capacitance and that is in the cup of the other unitand is interposed between the polarizing circuit output line leading tothe diaphragm therein and the transformer secondary thereof.

6. A headphone unit according to claim 5 wherein the polarizing circuitincludes a voltage multiplier network and a relatively high valueresistance element in each output line operative in combination with thecapacitance of the electrostatic driver to provide a time constant ofapproximately at least 0.1 sec.

7. A headphone unit according to claim 5 wherein the voltage multipliernetwork feeds a storage capacitor which is provided with a voltagelimiting means designed to limit the charge thereon to a predeterminedmaximum voltage.

8. A headphone unit according to claim 5 wherein the leakage inductivereactance of each transformer secondary and the input capacitance of itsdriver are tuned to resonate at a frequency above approximately 13 kHz.to enhance the high-frequency response of the speaker unit.

9. A speaker unit which comprises: an enclosure with a speaker opening;an electrostatic driver having opposite plates and a central movablediaphragm, said driver being mounted in the enclosure to face outwardlythrough the speaker opening and adapted to have its plates connected toan audio signal source; and a polarizing circuit adapted to be connectedto the same audio signal source to receive the audio signal and use itto generate a polarizing voltage for the diaphragm of said driver.

10. A speaker unit as recited in claim 9, wherein said polarizingcircuit comprises: a voltage multiplier circuit adapted to be connectedto receive the audio signal; a storage capacitor connected in shuntrelationship to the output of the source and the speaker unit, saidtransformer having a primary winding connected to the audio signalsource and a single secondary winding connected to the plates of thedriver and the voltage multiplier circuit.

PC4050 TED STA'lE PATENT @FHCE l oh unseen Patent No. 39 53 903 DatedJanuary 4, 1972 Inventofls) Martin L ng's, Jro

It is certified that error appears in the shove-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1 line 6, "an" (second occurrence) should be a.

Column 1,, line 24, "speakers" should be speaker Column 2,, line 18,,"circuits" should be circuit Column 2, line 21, M a should be insertedbefore "felt" Column 2,, line 70 "will" should. be with Column 3, line4, a should be inserted hefore "capacitor" Column 3, line 37, "32"should be 22 Column 3 line 74, "many" should be may Column 4,, line 25,"turned" should be tuned digned and sealed this 30th. day of May 1972..

(SEAL) Attest:

EDWARD MQFLETCHERMRQ ROBERT GOTTSGHALK Attesting Officer Commissioner ofPatents la. J

1. A headphone speaker unit comprising: an enclosure with a speakeropening; an electrostatic driver having opposite plates and a central,movable diaphragm, said driver being mounted in the enclosure to beadapted to face outwardly through the speaker openings; a couplingtransformer mounted in the enclosure that has a primary adapted to beconnected to an audio signal source and a secondary connected to thedriver plates; and a polarizing circuit leading from the secondary toprovide a polarizing voltage for the diaphragm, wherein said polarizingcircuit includes a voltage multiplier network having an input connectedto said coupling transformer secondary and an output connected to saiddriver diaphragm.
 2. A unit according to claim 1 wherein the polarizingcircuit includes a relatively high value resistance element connectedbetween the voltage multiplier output and driver diaphragm to provide atime constant of approximately at least 0.1 sec.
 3. A unit according toclaim 1 wherein the voltage multiplier network feeds a storage capacitorwhich is provided with voltage limiting means designed to limit thecharge thereon to a predetermined maximum voltage.
 4. A speaker unit asrecited in claim 1 wherein the leakage inductive reactance of thecoupling transformer secondary and the input capacitance of the driverare turned to resonate at a frequency above approximately 13 kHz. toenhance the high-frequency response of the speaker unit.
 5. Astereophonic headphone assembly for use with a stereophonic signalsource, said assembly comprising: a pair of headphone units each ofwhich includes a cup, an electrostatic driver that includes oppositeplates and a central diaphragm and that is mounted in the cup enclosureto be adapted to face a user''s ear, and a coupling transformer that hasa primary adapted to be connected to an audio signal source and asecondary connected to the driver plates, the primaries of the twotransformers being adapted to be connected to respective channels of thestereophonic source, the two transformers being provided with groundjumpers leading to a common ground connection; a polarizing circuit inthe cup of one unit that is connected to the transformer secondarytherein to generate a polarizing voltage and has output lines leading tothe diaphragms of both drivers; and an isolation netwoRk that includes aresistance and a capacitance and that is in the cup of the other unitand is interposed between the polarizing circuit output line leading tothe diaphragm therein and the transformer secondary thereof.
 6. Aheadphone unit according to claim 5 wherein the polarizing circuitincludes a voltage multiplier network and a relatively high valueresistance element in each output line operative in combination with thecapacitance of the electrostatic driver to provide a time constant ofapproximately at least 0.1 sec.
 7. A headphone unit according to claim 5wherein the voltage multiplier network feeds a storage capacitor whichis provided with a voltage limiting means designed to limit the chargethereon to a predetermined maximum voltage.
 8. A headphone unitaccording to claim 5 wherein the leakage inductive reactance of eachtransformer secondary and the input capacitance of its driver are tunedto resonate at a frequency above approximately 13 kHz. to enhance thehigh-frequency response of the speaker unit.
 9. A speaker unit whichcomprises: an enclosure with a speaker opening; an electrostatic driverhaving opposite plates and a central, movable diaphragm, said driverbeing mounted in the enclosure to face outwardly through the speakeropening and adapted to have its plates connected to an audio signalsource; and a polarizing circuit adapted to be connected to the sameaudio signal source to receive the audio signal and use it to generate apolarizing voltage for the diaphragm of said driver.
 10. A speaker unitas recited in claim 9, wherein said polarizing circuit comprises: avoltage multiplier circuit adapted to be connected to receive the audiosignal; a storage capacitor connected in shunt relationship to theoutput of the voltage multiplier circuit, which capacitor is charged toproduce the polarizing voltage; and a voltage limiter connected to limitthe magnitude of the polarizing voltage.
 11. A speaker unit as recitedin claim 10, which includes a coupling transformer connected between theaudio signal source and the speaker unit, said transformer having aprimary winding connected to the audio signal source and a singlesecondary winding connected to the plates of the driver and the voltagemultiplier circuit.